Alterations in the differentiated state of the smooth muscle cell (SMC) play a key role in the development and progression of a variety of cardiovascular diseases. The long-term goal of this project is to elucidate cellular and molecular mechanisms that control the growth and differentiation of SMC during vascular development, and how these control processes are altered during phenotypic switching of SMC in association with vascular injury and disease. Results of our recent studies have implicated the fascinating pluripotency gene Krupple-Like Factor 4 (KLF4) as a key mediator of SMC phenotypic switching in cultured SMC, and in vivo following carotid ligation injury in conditional KLF4 knockout (KO) mice generated in our lab. Studies in this proposal will test the hypothesis that KLF4 plays a critical role in regulating transitions in the differentiated state of vascular SMC during development, as well as during phenotypic switching following vascular injury or development of atherosclerosis. Aim 1 will determine mechanisms by which KLF4 represses expression of SMC marker genes in vivo with vascular injury including determining if KLF4 mediates repression SMC genes by: a) binding to conserved G/C repressor and/or TCE elements found in most SMC promoters; b) inducing epigenetic modifications associated with transcriptional silencing; c) suppressing expression of the potent SMC-selective SRF co-activator myocardin; and/or d) inducing pluripotency genes. Aim 2 will test the hypothesis that delayed repression of SMC marker genes and exacerbated lesion formation following vascular injury observed in our previous studies in conditional KLF4 knockout mice, were mediated, at least in part, by loss of KLF4 in SMC rather than its loss in EC or macrophages. Studies will include bone marrow transfer, and SMC specific conditional KLF4 KO studies in mice injury. Aim 3 is to determine if conditional or SMC specific conditional knockout of KLF4 alters atherosclerotic intimal lesion size or cellular composition in ApoE knockout mice. Aim 4 is to determine the role of KLF4 in activation of genes such as MMP3, MCP-1, iNOS, and VCAM-1 which are likely to play an important functional role in intimal lesion development and/or progression in response to vascular injury or experimental atherosclerosis.